Surfactant Formulations for Oil Recovery:
Surfactants normally contain predominantly hydrophilic groups and hydrophobic groups in two regions of the same molecule (FIG. 1). They have been proposed to facilitate recovery of crude oil from subterranean deposits [Gale et al., U.S. Pat. No. 3,946,812], even right after primary oil recovery operations. Improvements to surfactant-based oil recovery operations have been proposed and new methods are being discovered. Usually, surfactants are produced from natural materials (fatty acids, lignin, etc.) or by coupling small molecule reagents into oligomers along with modification reactions. An example of the latter is the formation of alkylaryl sulfonates, whereby a benzene-toluene-xylene (BTX) stream is functionalized with an alpha-olefin stream followed by the sulfonation of the benzene ring.
Early surfactant formulations proposed for oil recovery were based on conventional detergent materials in conjunction with water-flooding [Gale et al., U.S. Pat. No. 3,946,812; Farmer III et al., U.S. Pat. No. 3,943,160] and steam-flooding methods [Isaacs and Daniel, U.S. Pat. No. 4,458,759]. Other materials were proposed to improve the performance of surfactant flooding, such as alkali, co-surfactants, polymers and other chemicals [Gupta, U.S. Pat. No. 4,467,869; Chen and Williams, U.S. Pat. No. 4,577,000; Dardis, U.S. Pat. No. 4,509,597; Stapp, U.S. Pat. No. 4,470,461; Cooke, U.S. Pat. No. 4,460,791]. Other embodiments involve the application of these performance-enhancing agents, particularly the alkali and polymer materials, in separate injection slugs from the surfactant-bearing solution [Gupta, U.S. Pat. No. 4,467,869]. These embodiments have culminated with the so-called ASP technology, which involves the use of an alkali slug, followed by the surfactant slug, and then the relatively viscous aqueous polymer solution slug [Hsu and Hsu, U.S. Pat. No. 6,022,834]. Even though these surfactant-based methods can be very effective in experimentally recovering up to about 80% of tertiary oil-in-place (“OIP”), they involve heavy use of a variety of chemicals, some of them VOCs, which can lead to economic, environmental, and material compatibility problems.
Conventional fatty-acids-based alkylbenzene sulfonate (ABS) detergents proposed in oil recovery applications are strictly categorized as oligomers, since their molecular weights are lower than the so-called entanglement molecular weight, Me [Fried, 1995]. These materials normally have 18-26 alkyl groups, while polystyrene has an entanglement molecular weight of about 18,100 Daltons, corresponding to around 181 monomer segments. Poly(methyl methacrylate) (PMMA), on the other hand, has a value of Me equal to 5,900 Daltons, making it one of the more polar polymers, with a low entanglement molecular weight. Polyolefins, which are relatively nonpolar polymers, have still lower values of Me. Thus, a combination of MMA and olefin monomers can have Me values lower than 5,900 Daltons. The entanglement molecular weight of PMMA could be altered by incorporation of low entanglement molecular weight moeties with other monomers during polymer formation.
Broad molecular weight distribution (MWD) alkyaryl sulfonate surfactant macromolecules have been recently cited by Berger [Berger, P. D., U.S. Patent Application 20050199395] to result in the lowering of the interfacial tension (IFT) between oil and water compared to their equivalent narrow MWD counterparts. From a performance standpoint, this means that surfactant macromolecules of various sizes can form along interfaces of various radii of curvature, allowing the removal of large and smaller oil domains from a solid surface. Surfactant macromolecules usually have narrow molecular weight distributions, and thus there is a need to mix a number of surfactants from different reactor runs to enhance the performance of the mixture.
Mobility Control Operations:
Thickening agents have been added into surfactant-based oil recovery fluids in order to facilitate mobility control [Evani, U.S. Pat. No. 4,184,096; Pathak et al, U.S. Pat. No. 4,657,059; Boudreau, U.S. Pat. No. 6,776,234; Boudreau, U.S. Patent Application Publication No. 2004/0224854]. Materials that are popularly used are based on ionic polyacrylamides (“PAM”) and xanthan. Ionic PAM is obtained either by free-radical polymerization of acrylamide monomer with subsequent hydrolysis or by free-radical copolymerization of acrylamide and acrylic acid monomers followed by neutralization of the acrylic acid segments with sodium hydroxide. The degree of hydrolysis or fraction of ionizable groups ranges from 15% to 35%. For these thickening agents, weight average molecular weights are in the order of 107 Daltons, and polydispersity indices are in the 2-3 range. Xanthan, a biopolymer, is produced by fermentation of the bacterium Xanthamonas campestris. Weight-average molecular weights for this biopolymer are in the 4-5×106 Dalton range. Xanthan is monodisperse with polydispersity indices between 1.3 and 1.5. In water or brine solution, xanthan attains a double-stranded conformation stabilized by hydrogen bonds.
Thickener solutions in water have also been used as a flow diversion slug behind injection slugs of alkali and surfactant solutions. An obvious drawback of using conventional thickeners such as polyacrylamides or xanthan gum is the added cost to the oil recovery operation, which is equivalent to $5 to $10 per barrel of oil produced [Mohanty and Caneba, 2005]. None of the surfactant-thickener combinations takes advantage of cost savings by having the appropriate anionic surfactant also function as a thickener or having the thickener closely related chemically to the surfactant such that the combination can be produced from the same reactor or processing equipment. Moss, U.S. Pat. No. 7,125,825, describes an oligomer formulation wherein a surfactant is also a thickener. The hydrophobic portion of the surfactant is a branched or straight-chained saturated or unsaturated aliphatic hydrocarbon with the possibility of having hydroxyalkyl groups. The proposed thickened surfactant has viscoelastic properties. Claimed oilfied applications include hydraulic fracturing, gravel packing, and well completion. None of these applications includes the use of surfactant for oil displacement and recovery. Moreover, this reference cites disadvantages of polymer-based thickening, including compatibility problems with components resulting in chemical precipitation. Finally, thickeners of this reference were claimed to be pseudoplastic, a type that has reduced viscosity under shear, but increased viscosity when the shear is removed. Shpakoff et al., U.S. Pat. Nos. 7,137,447 and 7,055,602, also involves the mixture of an anionic aliphatic surfactant with an aliphatic nonionic additive for enhanced oil recovery performance. Here, both surfactant and additive are of oligomeric size.
Demulsifying Operations:
The action of surfactants as demulsifiers can be understood from an analogy with industrial cleaning operations, as described in a U.S. Environmental Protection Agency website [http://es.epa.gov/technifo/facts/florida/aque-fs.html]. Here, a surfactant/water solution is used to remove soil coated with machining oil from the surface of a metal part. A desirable surfactant is cited to be one that subverts the soil with oil from the part, rather than one that aggressively emulsifies the oil. This surfactant should have more affinity to the metal part than the soil covered with oil. From a processing standpoint, this weak surfactant system lifts the soil with oil and suspends it while the fluid mixture is being agitated. When the agitation is stopped, the oil separates from the surfactant-water solution and rises to the top, while the soil settles to the bottom, thus facilitating the reuse of the aqueous fluid. If an emulsifying surfactant is used, the oil and a portion of the soil will be suspended in the aqueous phase, resulting in a dirty fluid. Recycling the aqueous phase will require a separate operation involving the addition of a demulsifier. In terms of subterranean oilfied operations, tar sand recovery, and even oil clean-up from soil, the analogy calls for the use of surfactant molecules that contain polar hydrophobic portions which will have more affinity to the rock or soil surface than to the oil. Unfortunately, conventional surfactants contain only aliphatic and aromatic hydrophobic groups which have very good affinity to the oil; thus they act as emulsifying surfactants. That is why a separate and costly demulsification operation is done to separate and recover the oil from the surfactant [Amaud, U.S. Pat. No. 6,875,351; Van Den Berg et al., U.S. Pat. No. 6,787,027]. Otherwise, the surfactant can become completely incorporated into the oil, and can never be recovered.
Demulsification is an operation that is used in conjunction with conventional surfactant-based oil/bitumen recoveries. Oil/bitumen is normally well-dispersed within surfactant domains, and addition of a demulsifier is needed to free the oil/bitumen from the surfactant. Examples of commercial demulsifiers are polyol, amine, and resin products by Clearwater (Houston, Tex.), ALKEN® product line by Alken-Murray Corp., and Kernelix™ line by Uniqema. Other approaches to demulsification have been cited in the literature [Presley and Harrison, 1642824 August, 1972 DE 252/330; Deng et al, 2004; Newcombe, U.S. Pat. No. 4,216,079; Balzer, U.S. Pat. No. 4,842,067]. In order to facilitate the reuse of surfactant for oil/bitumen recovery, the surfactant would have to be separated from the demulsifier, which is normally a very difficult procedure, comprising liquid-liquid or solid-liquid extraction followed by distillation or vacuum stripping. If the surfactant has demulsification characteristics, then this separation may not be necessary, and surfactant reuse may become possible. This has been illustrated with oligomeric surfactants in Guymon, U.S. Pat. No. 5,252,138, which used a surfactant from the group consisting of a linear alcohol having carbon atoms within the range on the order of about eight to fifteen carbon atoms and ethylene oxide units on carbon atoms within the range on the order of about two to eight ethylene oxide units. However, no polymeric surfactants with demulsification characteristics have been identified for use in oil recovery and VOC loss control applications.
Oil Recovery from Surface Sources:
Methods for recovery of bitumen and fossil-based materials from tar sands and their tailings, shale oil, and surface/subsurface spills include thermal [Bouck, U.S. Pat. No. 4,412,585]; steam-assisted [Widmyer, U.S. Pat. No. 4,34,812; Needham, U.S. Pat. No. 4,068,717]; chemical [Hardin, U.S. Pat. No. 4,110,195; Mitchell, U.S. Pat. No. 4,410,551; Miller, U.S. Pat. No. 4,470,899; Graham et al, U.S. Pat. No. 4,722,782; Taylor, U.S. Pat. No. 4,822,481; Graham et al, U.S. Pat. No. 5,143,598]; and surfactant-based methods [Merchant Jr. and Smith Jr., U.S. Pat. No. 4,407,707; Siefkin and Boesiger, U.S. Pat. No. 4,368,111; Thirumalachar and Narasimhan Jr., U.S. Pat. No. 4,929,341; Guymon, U.S. Pat. No. 5,252,138; Olah, U.S. Pat. No. 5,000,872; Schramm and Smith, U.S. Pat. No. 5,009,773; Gregoli et al, U.S. Pat. No. 5,340,467; Ashrawi, U.S. Pat. No. 5,282,984; Catla, U.S. Pat. No. 5,746,909]. All of these methods can be implemented through strip-mining or excavation. In situ methods have also been proposed [Yildirim, U.S. Pat. No. 4,406,499]. Strip-mining or excavation is being implemented and has resulted in the alteration of the landscape, as well as enormous man-made lakes (such as Lake Mildred in Alberta, Canada) which, because they contain tailings from the bitumen extraction operation on the lakebed, are called tailings ponds. In situ extraction causes little or no disturbance to the landscape, but is an inefficient method. One in situ approach uses the steam-water-assisted-gravity (SAG) method for in situ recovery of bitumen from buried tar sands. Such an operation is energy intensive, since the steam is usually produced using natural gas.